Well, targetting your party members with a controller is really easy.
Use up/down to reach your partner in the list, press spell button... magic you're already back to your default target.
It's even easier if you sort your party member list

Targetting ennemies is easier too: with the L1/R1 switch or with L1+up/down through the enmity list.

I think the only situation where mouse users have an advantage is when healing alliance members.

Also, I'm not fond at all of the "L2 then R2" and "R2 then L2", cause it's messy, so I just add 8 extra slots with L2+R2.
That makes 24 slots, which is already enough.